I'm not a lawyer but, an C & D is a "scare letter" warning you they "can" or "may" do something (but not that they necessarily will do anything, which, depending on the case, may be very difficult). It's just an economical way for the sender to verbally put a lean on you to stop doing something. It puts you on notice that they are aware of activity associated with you, and that you are on their radar, usually due to a website or advertising you put up. And it tries to act as a 'fishing expedition' to make you admit to or pay something you don't have to, or which may not even be the case (e.g., if you are from a country which doesn't have intellectual property laws their claims may not apply to you, or be enforceable). Their demands are based on claims until they are established as fact, so you do NOT have to pay, disclose or confirm anything unless you are directed to by a judge.
In brief, whether you get a lawyer or not, take down your site/current ad (hopefully it was not in your name). If they try to sweat info from you, send them a response email or fax informing them you have ceased the activity, and are willing to supply information or pay them IF they agree to do A, B and C (like, agree to not pursue litigation, settle for $100 or less, that kind of thing). This is a conditional acceptance, and if they don't meet the conditions, then they are the party holding things up, and you don't have to disclose or pay. If they do pursue the matter further, you can then document you agreed to cooperate, but they didn't reciprocate or 'defaulted,' which will deeply weaken their position, if not cause their claims to be outright vacated. So leave it at a standoff, and if you must continue to do the activity, set things up more discreetly next time.
